Extreme Results
An optional rule, extreme rolls add drama. You can only get extreme results if you roll at least three dice. This does apply to Simple Pool if you want!
Extreme Failure
If all the dice roll zeroes you don’t just fail – you fail catastrophically. (You can’t get extreme fails without zero-based dice, but opposition can still get extreme successes against you.) Players are encouraged to suggest results, but the GM will make the final decision, and it should be something bad for the character. Not only do they utterly fail to accomplish anything good for that action, it has negative side effects.
It always means your turn is over. If you had other actions in progress, they are aborted. You cannot use a Luck Token, though you may still use actual Karma – but only to make it just a simple fail.
There should also be some immediate consequence. This should get more severe as the number of dice rolled increases. For just three dice, maybe the character fumbles and drops his weapon, lockpicks, reins, etc. As more dice are used, maybe the weapon breaks, or in extreme cases they get so off-balance they get no passive resistance rolls for the turn!
Extreme Success
The GM is within his rights to say that you can’t roll extreme successes without zero-based dice, since you can’t roll extreme fails, but it’s not a rule.
If all the dice are individually the highest value each could roll AND the total is exactly your EL, you have not merely succeeded, but done so spectacularly. Suggest results, but it should be something good for the character. Not only do they accomplish their best-possible result for that action, it has additional positive side effects.
Extreme successes always award a free Luck Token if the character has room. If opponents use a Luck Token to resist or oppose, get an immediate Karma.
There should also be some immediate beneficial result. This too should get more extreme as the number of dice rolled increases. For just three dice, maybe the character disarms their opponent, or creates a Boost for the next roll in addition to their intended effect. As more dice are used, maybe they get to assign the results as a split (see Multiple Actions) after the roll, or in extreme cases their target gets no opposition or passive resistance at all!
Observations
Note that if you are using AnyDice unconstrained rolling, or even just Simple Pool Extra Effort dice, the player can and often will choose dice pools that do not total up to exactly their EL if every die rolls its highest result. This lets them manipulate their odds of getting better rolls at a certain amount of self-chosen risk, but it means they cannot get extreme successes on those rolls. They can still get extreme fails if they are using zero-base dice!
It is also worthy of note that as characters become more competent they become less likely to get extreme rolls of any kind. This is by design. Extreme rolls are accidents of chance – a fumble, or a fortuitous windfall. They aren’t the result of skill. Skillful rolls are more dice, and optimal results are achieved by foregoing extreme successes with extra effort. You never have to play with that sort of dice-fiddling, but it’s an available tactical choice.